Workpiece supply fixture for induction heating apparatus



July 10, 1956 w. B. ALBRECHT WORKPIECE SUPPLY FIXTURE FOR INDUCTIONHEATING APPARATUS Filed June 5, 1953 2 Sheets-Sheet l July 10, 1956 w.B. ALBRECHT 2,754,396

WORKPIECE SUPPLY FIXTURE FOR INDUCTION HEATING APPARATUS Filed June 5,1953 2 Sheets-Sheet 2 WORKPIECE SUPPLY FIXTURE FQR INDUCTION HEATINGAPPARATUS William B. Albrecht, Wauwatosa, Wis, assignor to Allis-Chalmers Manufacturing Company, Milwaukee, Wis.

Application June 5, 1953, Serial No. 359,744

9 Claims. (Cl. 21910.69)

This invention relates in general to induction heating apparatus andmore specifically to an improved fixture for automatically supplyingworkpieces to the heating station in a consecutive manner.

The heat treating of various kinds of objects by a high frequencycurrent is well known and has proved successful in producing thereindesired metallurgical characteristics. The object to be heated, commonlyreferred to as the workpiece, is positioned in a heating station,defined by a portion of an induction heating work coil, for apredetermined period of time, depending on the size and shape of theworkpiece and the heat treating operation being performed.

In some suggested apparatus the work coil which defines the heatingstation comprises a hairpin like inductor, the workpieces passingthrough the inductor in a direction parallel to the lengthwise directionof the inductor while they are being heated. In other suggestedapparatus the work coil comprises a number of conductor turns whichdefine a vertical passage, the workpieces being positioned in thepassage so that the conductor turns substantially surround it during theheating cycle. Of the two types of work coils it has been found that thework pice is heated more uniformly when it is substantially surroundedby the work coil.

Previously suggested induction heating apparatus which utilized a workcoil that substantially surrounds the workpiece have not provedsatisfactory for some types of heat treating operations. Specifically,in annealing thin wall cylindrically shaped workpieces, such as shellcastings where the time required to heat the workpiece is relativelyshort, the apparatus does not supply workpieces to the work coil withsufficient speed to operate efificiently.

To overcome this disadvantage it is proposed, according to the presentinvention, to provide an improved induction heating apparatus whichincludes a fixture for positioning workpieces rapidly in the heatingstation. This fixture comprises a pair of substantially parallel fiatmovable plates disposed transverse to and on opposite ends of theheating station. One plate provides a support for the workpiece in thestation and the other plate provides a support for the workpieces topositioned in the station. The plates cooperate with each other on beingmoved to allow the workpieces to be fed by gravity to the heatingstation in a consecutive manner. in addition to supporting the workpiecein the heating station the lower plate provides means for rotating theworkpiece to insure an even distribution of heat.

It is therefore an object of the present invention to provide animproved induction heating apparatus for heat treating workpieces in aconsecutive manner.

Another object of the present invention is to provide an improvedinduction heat treating apparatus which includes a fixture forsimultaneously supplying workpieces in a consecutive manner to aplurality of heating stations.

A further object of the present invention is to provide an improvedinduction heating apparatus including a fixture which automaticallypositions the workpiece in the States Patent 2,754,396 Fatented July 10,1956 heating station and rotates the workpiece while it is being heated.

A still further object of the present invention is to provide aninduction heating apparatus for consecutively annealing a plurality ofthin wall cylindrical workpieces, the apparatus including a fixturehaving a pair of movable members which cooperate with each other toallow the workpieces to be positioned rapidly by gravity in the heatingstation in a consecutive manner.

Objects and advantages other than those mentioned above will becomeapparentfrom the following description when read in connection with thedrawings in which:

Fig. 1 is a front view partly in section of the induction heatingapparatus embodying the present invention;

Fig. 2 is a partial plan view of the apparatus shown in Fig. 1; and

Figs. 3 and 4 are schematic views of the apparatus shown in Fig. l.

The apparatus shown in Fig. 1 comprises generally a source of highfrequency current shown schematically as a generator 12 which isconnected to an induction heating work coil 13 for heat treatingworkpieces being supplied to work coil 13 by fixture i4.

Generator 12 and work coil 33 may be of any suitable type known in theart. As shown, work coil 13 comprises a plurality of fiat plateinductors 17, i8 and 19 which are disposed in parallel spacedrelationship with each other and which are connected togetherelectrically by cooling conduit 2% in a suitable manner. The inductorsare provided with two cylindrical openings each, which form two passages21 and 22 when the inductors are assembled with corresponding Openingsin axial alignment.

Fixture 14, which operates to position workpieces automatically inpassages 21 and 22, comprises a frame having a base 23 and a panel 24disposed substantially perpendicular to base 23 and secured thereto bybraces 26, 27. A shelf 23, disposed parallel to base 23 and rigidlysecured to panel 24 in a suitable manner, serves to support a holdingdevice 29 and a control switch 30 for generator 12. A centrifugal switch31 is mounted on a top flange 25 positioned above shelf 28. Device 29and switches 34) and 31 are all actuated by rotation of shaft 32 whoseaxis of rotation is parallel to the axes of passages 21 and 22 of workcoil 13 and is spaced from each axis a predetermined distance. Shaft 32is suitably journaled in portion 33 of bracket 34, which is rigidlybolted to panel 24.

Shaft 32 may be actuated by any suitable known means to cause it toeither rotate or oscillate about its axis. In the preferred embodimentof the apparatus, shaft 32 is rotated about its axis by means of avariable speed motor 36 which has its shaft 37 connected to shaft 32through a spring biased slip clutch 38.

A plate 39 is disposed on shaft 32 immediately below shelf 28 and isfixedly attached to shaft 322 by a key 41. Plate 39 when suitably movedto one position operates to support two columns of workpieces which arestacked in vertically disposed guide tubes 48 and 49, portions of whichare shown in Fig. 1, and when plate 39 is moved to another position itis inoperable to support the workpieces. Guide tubes 48 and 49 arepositioned above passages 21 and 22, respectively, of work coil 13 andaligned therewith so that the columns of workpieces rest on plate 39,the columns extending upward through openings 51, 52 in holding device29 into guide tubes 48, 49 which are attached to device 29. In order toallow workpieces to be supplied to the heating station by gravity plate39 is provided with an opening through which the lowermost workpiecesmay pass when the plate is suitably actuated. If desired a plurality ofsuch openings may be provided in the plate, the openings being spacedcireumferentially .withrespect to each other. If the work coil hasaplurality of heating stations the plate may also be provided with otheropenings which are in radial alignment with the circumferentially spacedopenings. As

shown-in Eig.,3 plate 39 is substantially circular and is provided withtwo circumferentially elongated openings 42 and43 which-are disposeddiametrically opposite each other and are located with 'theircenterlines an equal distance from the center of plate .39. Also since workcoil 13 :has;two heating stations, the circumferential edge 4412f plate,39 is recessed radially inward oppositeopenlugs ;42 and 43 to providetwo othertopenings 416 and :47 which are radially aligned withopenings42, .43, respectively.

.Assecond plate 53 is spacedaxiallybelowplate 39 cm shaft 32'and'fixedly attached to the latter by. a set screw 54. :Platei53 is similarto plater39 andoperates to sup port :the workpieces inpassages21 and22of work coil 13 during theiheatingcycle. As shown in Fig. 3 plate 53differs from plate 39 in that it has twoannular recesses 56 and 57.'Recess 56 forms a-pair of circular edges 58 on the upper face of plate53 and recess 57 forms alike pair of edges 59. One edge of each pairliesson circles which are concentric with shaft 32 and pass through theaxes of passages 21 and 22. When plate 53 is rotated recess 56 passesunder only the half of passage 21 which is nearest the axis of rotationof shaft 32, the radius 'ofone of the circular edges-58 being equalto'the distance between the axes of shaft 32 and'passage '21. Recess 57issimilarly positioned with respect to passage 22. 'Therefore, while theworkpieces are being supported in passages '21, 22 by plate 53, onlyhalf of the workpieces are in contact with the plate atany one'timeytheotherhalf'extending over one of the annular recesses 56-or 57.

"Disposed intermediate plate 39 and plate 53 is a U-shaped channel 61which is secured to bracket 34 and which serves as a holding means forceramic bushings 62. Ceramic bushings 62 are disposedon both ends ofpassages'21 and 22 of work coil 13 and are fitted into suitable recessesof channel 61. Thesetbushings when axially aligned with passages 21 and'22 act aslgnides for-centering the-workpieces in the passages and forpreventing'the workpieces from coming in contact with any part of workcoil 13.

"Holding device 29 operates .to hold each column of workpieces-in afixed position when plate 39 is rotated to a nonsupporting position, forexample, when any of t'heopeningsin plate 39 are positioned belowpassages 51, 52'of-device29. Device 29 comprises an actuating arm :63having one end pivotally attached to a ,pivot pin 64 secured to topflange 25, and the other end operable'to hold the'columns of workpiecesstationary. Since the diameters of the workpieces may vary slightlymeans are-provide'd-to-allow one column to beheld with aforceindependent of the force on the other column. This holdingaction isobtained through spring 66 which biases arm 63 against cam,,67 that ispositionedon. shaft32in apre'determined relationship with platet39.The-endof arm 63-which'acts to clamp the second lowermost workpiece 68of-each column againstthe walls .of openings 51 and 52, includes twopins .71 and '72 whichjbear againstthe workpieces 68 when arm 63isint'heholding position, pin 71 being rigidly fixed in arm .63.whilepin 72 isbiasedI'by-a spring 73. This arrangementallows, theworkpiece-*inteach column to beheldjfirmlylbecausethe holding'force'of.pins 771, 72 are independent of each other. When plate 39 is in aposition to s,upp.ort .the column of workpieces, cam ,67 :opposesthebiasingr-actionsof spring'66-and allows the workpiecesto drop; and.rest ;on p'late39. I

--'Control switch 130 is actuated ,by a cam 76 which .is suitablypositioned on shaft "32 to open switch 30 and cause =work -coil 13 tobe-energized whenthe workpieces arcvpositione'd in passages 21 "and "22:of work coil 13.

Cam 76 is a butterfly type cam and may be adjusted to .va y th timeswitch 30 remains n h open posit on- Likewise, centrifugal switch 31 ismounted on top flange 25 and suitably coupled to shaft 32 by a chaindrive to prevent work coil 13 from being energized by switch 30 whileshaft 32 is rotating below a predetermined speed.

The various electrical connections for the apparatus are shownin Figs. 3and 4 which represent Schematic views of the apparatus shown in Fig. 1.Motor 36 is connected to a suitable voltage supply circuit 79 through amanually operated switch 80. Generator ,1'2'is also connected acrosscircuit 79 through manually operated switch 81. Control switch 30 isconnected to generator 12 through suitable control means 83, andcentrifugal switch 31 is connected in shunt with switch 30. Controlmeans 83 operates to cause work coil 13 to be energized from generator12 only when both switches 30 and 31 are open.

The operation of the apparatus, shown in Fig. l in the deenergizedposition, is as follows. Switch 81 is move to the closed position toenergize generator 12, and switch 89 is moved to the closed positiontoenergize'motor 36 and cause shaft 32 to rotate clockwise, as shown inthe drawings. Rotation of shaft 32 opens centrifugal switch 31 whichallows the energizing of work .coil 13 to be controlled by switch 39.

As shown in Fig. 1 the stacks of workpieces in guide tubesdii and 49 aresupported by plate 39 and remain supported by the plate untilimmediately before the openings idZ and 46 in plate 39 are positionedbelow passages 51 and 52 in device 29, at which time arm 63 is moved tothe clamping position by action ofbiasing spring 66 and cam 67. Arm 63when in the clamping position causes pins 71 and 72 to force the secondlowermost workpiece 6'8 of each stack against the walls of passages 51and '52 thereby holding the stack of workpieces in a-stationaryposition. Arm 63 remains in this position as shaft 32 continues torotate causing openings 42 and 46 of plate 39 to pass under passages 51and 52 which allows the lowermostworkp-iece 89 in each stack to dropinto passages *21 and 22.

Plate 53 is positioned on shaft 32 with its openings circumferentiallyoffset from corresponding opening'in plate 39 so'that by thetimethe'bottom of the workpieces 89 have reached the plane of plate 53,the latter is in a position to supportworkpieces -89 in passages 21 and22. This condition is shown schematically in Fig. 3 where the Workpieces89 which were lowermost in each tube have fallen through the radiallyaligned openings 42 and 46 in plate 39 into passages 21 and 22. Cam 67is shown about to actuate arm 63 to the nonclarnping position and allowthe stacks of workpieces to drop and be supported as before by plate 39.Control switch 30 as shown in Fig. 3 is actuated to the open position bycam 76 ,thus allowing work coil 13 to be energized from generator 12. 7

If desired, workpieces 89 presently positioned in passages 21 and 22 maybe rotated about their respective axes by suitable means while beingheated. As shown in Figs. 3 and 4 only one-half of the bottomof eachworkpiece-89 is in sliding contact with the plate at any one timebecause the other half extends ,over onetof the annular recesses 56 or5'7. Therefore, the friction between the bottom of the workpieces 68 andthe portions of the plate adjacent the annular recesses 56, 57 causeseach workpiece to be turned clockwise about its axis. T 0 provide morerotation of the workpiece the .coefiicient of friction of thecontacting'portion may be increasedinany suitablemanner, such as byknurling the contacting portions or inlayinga piece of material having ahigher coe'flicientof friction than the plate. I

At theend-ofthe heating cycle plat e,53 hasheenn'ptated to'a position sothat openings 43and47 of plate 53 are passed under paS. s 21, 2 2tofwork coil .13 ftoallow the heated workpieces to ,drop thIQughjhe5e gp emThis conditionis shownschematically in,Eig.i4 Where the heatedworkpieces are beginning to drop from passages 21 and 22, control switch36 is then in the closed position and arm 63 has just been moved to theclamping. position by cam 67 and spring 66. Further rotation of shaft 32brings the plates to a position similar to that shown in Fig. 3, butwith shaft 32 being rotated 180 soas to position openings 43 and 47where openings 42 and 46 are positioned in Fig. 3. The operation is thenrepeated.

If for some reason plate 39 or plate 53 becomes jammed by a workpiece sothat shaft 32 cannot rotate, clutch 38 allows motor shaft 37 to continuerotating. However, since centrifugal switch 31 which is actuated byrotation of shaft 32, is then closed, work coil 13 will not be energizedand the danger of overheating a workpiece is eliminated.

The apparatus is shown in the drawings utilizing only one work coil thatdefines two heating stations. However, it will be apparent to oneskilled in the art that a plurality of work coils may be used in spacedrelationship with each other if desired, and it will also be apparentthat the work coil may define one or more heating stations.

Although but one embodiment of the present invention has beenillustrated and described, it will be apparent to one skilled in the artthat various changes and modifications may be made therein withoutdeparting from the spirit of the invention or from the scope of theappended claims.

It is claimed and desired to secure by Letters Patent:

1. Apparatus for consecutively heat treating a plurality of workpiecesby induction from a high frequency current, said apparatus comprising awork coil defining a heating station; means for positioning workpiecesin said station in a consecutive manner comprising means dis-- posedabove said station for stacking said workpieces, a first movable memberdisposed above said station and below said stacking means, said firstmember being operable in one position to support the lowermost workpieceof said stacked workpieces and inoperable in another position, a secondmovable member disposed below said sta-. tion, said second memberoperable in one position to sup-. port a workpiece in said station andinoperable in another position, mechanical means common to both membersto move said members from said one position to said other position tocause said workpieces to be positioned in a consecutive manner bygravity in said heating station; means for energizing said work coilwith a high frequency current and means to control said energizing means.in a predetermined manner with respect to said positioning means.

2. Apparatus for consecutively heat treating a plurality of workpiecesby induction from a high frequency cur' rent, said apparatus comprisinga work coil defining a heating sttaion; means for positioning workpiecesin said station in a consecutive manner comprising means disposed abovesaid station for stacking said workpieces, a first movable memberdisposed above said station and below said stacking means, said firstmember being operable in one position to support the lowermost workpieceof said stacked workpieces and inoperable in another position, a secondmovable member disposed below said station, said second member operablein one position to sup-. port a workpiece in said sttaion and inoperablein anotherposition, an actuating shaft connected to move said membersfrom said one position to said other position to cause said workpiecesto be positioned in a consecutive manner by gravity in said heatingstation, and means to move said shaft; means for energizing said workcoil with a high frequency current and means to control said energizingmeans in a predetermined manner with respect to said positioning means.

3. Apparatus for consecutively heat treating a plurality of workpiecesby induction from a high frequency current, said apparatus comprising awork coil having a workpiece receiving passage; means for positioningworkpieces in said passage in a consecutive manner, said positioningmeans including a fixture comprising an actuating shaft having its axisof rotation parallel to the axis of said workpiece receiving passage andspaced laterally therefrom a predetermined distance, means to rotatesaid shaft, a first plate disposed above said passage and fixedlyattached to said shaft, a second plate disposed below said passage andfixedly attached to said shaft, each said plate having an openingdisposed therein spaced radially from said of rotation of said shaftsaid predetermined distance, and means disposed above said first platefor stacking said workpieces along said axis of said passage with thelowermost workpiece in contact with said first plate, said plates beingdisposed on said shaft so as to cooperate with each other on rotation ofsaid shaft to cause each said lowermost workpiece to be positionedconsecutively in said passage; means to energize said work coil with ahigh frequency current; and means to control said energizing means in apredetermined manner with respect to said positioning means.

4. Apparatus for consecutively heat treating a plurality of workpiecesby induction from a high frequency current, said apparatus comprising awork coil having a workpiece receiving passage; means for positioningworkpieces in said passage in a consecutive manner, said positioningmeans including a fixture comprising an actuating shaft having its axisof rotation parallel to the axis of said workpiece receiving passage andspaced laterally therefrom a predetermined distance, means to rotatesaid shaft, a first plate disposed above said passage and fixedlyattached to said shaft, a second plate disposed below said passage andfixedly attached to said shaft, each said plate having an openingdisposed therein spaced radially from said axis of rotation of saidshaft said predetermined distance, means disposed above said first platefor stacking said workpieces along said axis of said passage with thelowermost workpiece in contact with said first plate, said plates beingdisposed on said shaft so as to cooperate with each other on rotation ofsaid shaft to cause each said lowermost workpiece to be positionedconsecutively in said passage, and means cooperating with said secondplate to cause rotation of said workpieces positioned in said passagewhen said work coil is energized; means to energize said work coil witha high frequency current; and means to control said energizing means ina predetermined manner with respect to said positioning means.

5. Apparatus for consecutively heat treating a plurality of workpiecesby induction from a high frequency current, said apparatus comprising awork coil having a workpiece receiving passage; means for positioningworkpieces in said passage in a consecutive manner, said positioningmeans including a fixture comprising an actuating shaft journaled bysaid fixture with its axis of rotation parallel to the axis of saidworkpiece receiving passage and spaced laterally therefrom apredetermined distance, means to rotate said shaft, a first circularplate disposed above said passage and fixedly attached to said shaft, asecond circular plate disposed below said passage and fixedly attachedto said shaft, each said plate having a circumferentially elongatedopening disposed therein and spaced radially from said axis of rotationof said shaft for said predetermined distance, a guide tube disposedabove said first plate for stacking said workpieces along said axis ofsaid passage with the lowermost workpiece in contact with said firstplate, said plates being disposed on said shaft so as to cooperate witheach other on rotation of said shaft to cause each said lowermostworkpiece to be positioned consecutively in said passage; means toenergize said work coil with a high frequency current; and means tocontrol said energizing means in a predetermined manner with respect tosaid positioning means.

6. Apparatus for consecutively heat treating a plurality of workpiecesby induction from a high frequency current, said apparatus comprising awork coil. having a. workpiece receivingpassage; means .for positioningworkpieces in said passage in a consecutive manner, said positioningmeans including a fixture comprising an actuating shaft journaled bysaid fixture with. its axis. of rotation parallel to the axis of saidworkpiece receiving passage and spaced laterally therefrom apredetermined distance, means to rotate said shaft, a first circularplate disposed above said passage and fixedly attached to said shaft, asecond circular plate disposed below said passage and fixedly attachedto said shaft, each said plate having a circumferentially elongatedopening disposed therein and spaced radially from said axis of rotationof said shaft for said predetermined distance, a guide tube disposedabove saidfirst plate for stacking said workpieces along said axis ofsaid passagewith the lowermost workpiece in contact with said firstplate, said plates being disposed on said shaft so. as to. cooperatewith each other on rotation of said shaft to cause each said lowermostworkpiece to bepositioned consecutively insaid passage, and means tohold said workpieces in stacked position while said lowermost workpiece.is being positioned in said passage; means to energize said work coilwith a high frequency current; and means to control said energizingmeans in a predetermined manner with respect to said positioning means.

7. Apparatus for consecutively heat treating a plurality of workpiecesby induction from a high frequency current, said apparatus comprising awork coil having a workpiece receiving passage; means for positioningworkpieces in saidpassage in a consecutive manner, said positioningmeans including a fixture comprising an actuating shaft journaled insaid fixture with its. axis of rotation parallel to the axis of saidworkpiece receiving passage and spaced laterally therefrom apredetermined distance, means to rotate said shaft, a first circularplate disposed above said passage and fixedly attached to said shaft, asecond circular plate disposed. below said passage and fixedly attachedto said shaft, each said plate having a circumferentially elongatedopening disposed therein. with its center spaced radially rom said axisfor said predetermined distance, means for stacking said workpiecesalong the axis of said passage above said first plate with the lowermostworkpiece in contact with said first plate, said plates being positionedon said shaft with respect to each other so as to cooperate with eachother onrotation of said shaft to cause each said lowermost workpiece tobepositi'oned consecutively in said passage, means to hold saidworkpieces inv stacked position while'said lowermost'workpiece is beingpositioned in said passage, and means to cause rotation of saidworkpiece. positionedinsaid passage comprising an annular recessdisposed in said second plate concentric with said shaft to form tworadially spaced circular edges on the upper face of said second plate,the radius of one of said circular edges being at least equal to saidpredetermined distance; means to energize said workcoil with a highfrequency current; and meanscto control said energizing means apredetermined relationship with said positioning means.

8. Apparatus for consecutively heat treating a plurality of workpiecesby induction from. a high frequency current, said apparatus comprising awork coil having a plurality of workpiece receiving passages; means forsimultaneously positioning workpieces insaid passages in a consecutivemanner, said positioning means including a fixture comprising anactuating shaft suitably journaled in said fixture with its axis ofrotation parallel to the axes of said workpiece receiving passages. andspaced laterally from one of saidpassages a predetermined distance,means to rotate said shaft, afirst circular plate disposed above saidpassages. and fixedly attached to said shaft, a second circular platedisposed below saidpassages and fixedly attached to. said shaft, eachsaid plate having apair of radially spaced circumferentially elongatedopenings having, radial widths sufiicient to allow workpieces to passthrough said openings unobstructed when said openings are aligned withsaid passages, means for stacking said workpieces along the axes of saidpassages. above said first plate with the. lowermost workpieces incontact with said first plate, said plates. being positioned on saidshaft with respect to .each other so .as to cooperate with each other onrotation of said shaft to. cause said lowermost workpieces to bepositioned in said passages, means to hold said workpiecesin stackedposition while said lowermost workpieces are being positioned in saidpassages, means. to energize said workcoil with a high frequencycurrent; and means to. control said energizing means in a predeterminedrelationship with said positioningmeans.

9... Apparatus. for consecutively heat treating a plurality ofworkpieces by induction from a high frequency current, said apparatuscomprising, a work coil having a plurality of workpiece receivingpassages; means for simultaneously positioning workpieces in said.passages in a consecutive. manner, said, positioning means including afixture comprising anactuating shaft suitably journaled in said fixturewith its axis of rotation parallel to the axes of said workpiece.receiving passages and spaced laterally from one of said passages apredetermined distance, means to rotate said shaft, afirst circularplate disposed. above said passages a-nd fixedly attached. tosaid-shaft, a second circular plate disposed below said pmsages andfixedly attached to said shaft, each'sa-id plate having a pair ofradially spaced circurnferentially elongated openings having radialwidths suflicient to. allow workpieces to pass through said openingsunobstructedwhen said openings are aligned with said passages, means forstacking said workpieces along the. axes of said-passages. above'saidfirst plate with the lowermost workpiecesincontact with said firstplate, said plates being-positioned on said shaft with respect to eachother so-asto cooperate with each other on rotation of said shaft tocausesaid lowermost workpieces to bepositioned in said passages, meansto hold said workpieces instacked position'. while said: lowermost:workpieces are being positioned insaid passages, 31111211163118 to causerotation of said workpiecespositionedq in said passagescomprisingannular recesses; disposed'in said second plate concentric, with saidshaft to. form radially spaced circular edges on the upper face of saidplate, said radially spacing of two of said edgesbeingequalto the radialspacing of said elongated openings andthelradiusof one of said circularedges. being" equal to; said predetermined distance; means to energizesaid work coil with a high frequency current; and means. to. controlsaid energizing means in a'predetermined relationship with saidpositioningmeans.

References. Cited in the file of this patent UNITED STATES PATENTS1,776,697 McGuinness et a1. Sept. 23, 1930' 1,784,967 McGuinness Dec.16, 1930 2,598,015 Richardson May 27, 1952 2,604,576 Collyer July 22,1952 2,647,199 Wharfi July 28, 1953

